The sophistication and capability of automobiles is expanding exponentially as automobile manufactures race to differentiate themselves in the market place by adding features and technological capabilities to their products. As the cost, size and power consumption of certain technological components drops, these components become feasible additions to automotive systems. This is particularly so of sensors that allow new functionality to be included.
One such sensor that is gaining popularity is the use of radar in automobiles. Although radar has been in existence since before World War II, the size and power consumption of it various components and the rotating nature of the send/receive antenna made such use in automobiles stuff for science fiction novels. However, with the adoption of the SPY-1 phased array radar system by the U.S. Navy 25 years ago, fully solid state, non-rotating antenna radar systems for automotive use became possible.
Unlike a classic rotating radar antenna, a phased array radar antenna is a composite antenna composed of multiple transceiver elements, each of which is controlled by a phase shifter. Radar beams are formed and directed by shifting the phase of a signal emitted from each element so as to create a constructive and destructive interference pattern that can be “steered” in the direction of the increasing phase shift among the elements, without having to physically redirect any element of the composite antenna. The constructive pattern steers the beam while the destructive pattern improves the sharpness/resolution of the beam. Some exemplary phased array systems that are suitable for automotive uses include 25 GHZ multi-mode radar systems sold by Autoliv, Inc. of Stockholm Sweden and 76 GHz radar systems from both Delphi Automotive PLC of Troy Mich. and the Bosch Group of Stuttgart Germany.
As simple examples, phased array radars may have linear arrays of radiating/receiving elements or planar arrays of radiating elements. Linear array radars feature rows of radiating/receiving elements in an X-Y matrix, where each row is controlled by a common phase shifter. Linear arrays may only steer the composite radar beam in one direction. Planar arrays feature radiating elements in an X-Y matrix where each radiating/receiving element has its own phase shifter and thus can be coordinated by computer to “steer” the composite radar beam in two dimensions. Phase shifters may operate to both “steer” a beam being transmitted by the antenna array and to “steer” the sensitivity of the antenna array to look in a particular direction for a return signal being received.
Like all radars, a phased array radar antenna does not transmit a single clean, monolithic radar lobe. Because of the constructive/destructive patterns, smaller lobes on either side of the main lobe exist. In many cases, the side lobes are undesirable and efforts are made to suppress their size and power because they are a source of ambiguity in regard to precisely locating a close-in radar contact. However, they will always exist.
Many parameters of an array affect its overall radiation pattern, including the number of elements, spacing between the elements and the digital weighting used to combine the energy from each of the individual elements. Any or all of these parameters could be employed to achieve the variation in main lobe power-to-side lobe power ratio.
The overall width of the main beam of an antenna array is most directly determined by its electrical size. The larger the antenna, the narrower the main beam. This electrical size can be varied by either physically varying the number of elements in the array and/or physically varying the spacing between elements.
The side lobe structure of an antenna is most directly determined by the number of elements that make up the array and their electrical spacing, so for a given element spacing, as the number of elements is changed, the number of side lobes will vary along with their positions. As the number of radiating/receiving elements decreases, the main lobe of the phased array radar gets wider and the number of side lobes decreases. For example, if there are only two radiating/receiving elements in a matrix, there will be two nulls in the beam pattern thus producing a main lobe and two side lobes. The fewer the number of radiating elements, the more pronounced the side lobes and the less pronounced is the main lobe.
Further, amplitude weighting used to combine the energy from each of the elements can also be used to vary the main beam to side lobe level ratio. A uniform element weighting will achieve the narrowest main beam. However, by reducing the weighting of the outer elements relative to the inner array elements, a higher peak side lobe-to-main lobe ratio will result along with a wider main beam.
FIGS. 1A-1C present explanatory diagrams (A-C) of arrangements of an exemplary 8×8 phased array matrix of elements 11. An 8×8 array 10 (See, FIG. 1A) will produce a relatively strong, narrow, well defined main lobe 100 with eight small side lobes 150 radiating alongside the main lobe in both elevation and in azimuth at progressively wider angles from the main lobe (See, FIGS. 2 and 3). In FIG. 1A, by ceasing radiation from all of the radiating elements 11 except for an 8×2 subset matrix 20, the main lobe 100′ is weakened relative to is former condition 100 and more energy is radiated by side lobes 150′ in the elevation or Y direction (See, FIG. 2). In FIG. 1B, by ceasing radiation from all of the radiating elements 11 except for an 2×8 subset matrix 30, the main lobe 100′ is weakened relative to is former condition 100 and more energy is radiate in only two side lobes 150′ in the azimuth or X direction (See FIG. 3). Similarly, in example C, ceasing radiation from all of the radiating elements 11 except for a 2×2 subset matrix 40, the main lobe 100′ is weakened even further relative to its original condition 100 and more energy is radiated by the side lobes in both azimuth and elevation.
The matrix of elements can be altered by reconfiguring sub-arrays in the matrix. For example, in a reconfigurable sub-array approach, if the antenna matrix has 9×9 elements, the active matrix may be constructed with 3×3 sub-arrays. Alternatively, if digital beam forming is an option, a system designer may sum individual elements to construct the active matrix.
Other radar antenna types that may be adapted to automotive use include conventional parabolic dish antennas and digital beam forming antennas. Digital beam forming is the combination of radio signals from a set of small non-directional antennas to simulate a large directional antenna. The simulated antenna can be pointed electronically without using phase shifters. In beam forming, both the amplitude and phase of each antenna element are controlled. Combined amplitude and phase control can be used to adjust side lobe levels and steer nulls better than can be achieved by phase control alone.
An automobile is relatively small size, thus its proximity to the ground and its proximity to obstacles (such as other automobiles) often results in an automotive main lobe being blocked. However, it is difficult to electronically disambiguate a situation where the radar beam is blocked by an obstacle from one where there are no obstacles to be detected. Further, nearby adjacent vehicles may be detected by the side lobes 150, thereby causing a false indication that the vehicle is in the main lobe 100. Hence, it is desirable to minimize the bad effects of side lobes and to use the existence of side lobes to advantage where they do exist to improve the detection of obstacles by the automobile.
Furthermore, other desirable features and characteristics will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.